320 MOEPHOLOGY AND EVOLUTION
neopallium, as in the case of Man, will enable the animal to survive and retain primitive features.
It must be strongly emphasized that two animals which retain several identical primitive features are not necessarily related in consequence, And systems of classification which ignore this cardinal principle are valueless.
When two animals, which are quite unrelated, come under the influence of the same physiological conditions some of their organs exhibit similar characters, for they have to function almost identically. They undergo what is known as convergent evolution, and convergent characters are frequently seen in the teeth, tongue and stomach. It is, however, easy to attribute too much to convergence, likewise to parallel evolution.
It is essential for the student to remember that the geological record is very incomplete, and the discovery of new fossils sometimes leads to a readjustment of our views of phylogeny. It has revealed many branches of the tree of life, and several ancestral forms from which group's of modern animals have arisen. At present we must postulate several hypothetical animals to fill gaps in the record, and wait for the discovery of actual fossils which take their places and give us a complete account of the evolution of living animals.
It is now established that Mammals have arisen from one of the families of the Cynodont Reptiles. The early Mammals, according to Huxley (&9), constituted the Prototheria, from which the Monotremes were derived. These Prototheria gave way to Metatheria from which the Marsupials were evolved. And the Metatheria gave Vay to £he Eutheria, from which the placental Mammals